Move opaque to 2nd instead of the 2nd to last, so that compilers help
check with the conversion.

Signed-off-by: Fam Zheng <famz@redhat.com>
Message-Id: <20170421122710.15373-7-famz@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
[Commit message typo corrected]
Signed-off-by: Markus Armbruster <armbru@redhat.com>